Codec Not Found...

While working on a scene in the middle of my project, I decided to crop the video with the wireframe tool, but when I told FC to render a message popped up saying, "Codec not found. You may be using a compression type without the coresponding hardware card."

I tried turning everything off and then on, and that didn't work. I then tried rendering other parts of the sequence, and it worked but the message popped off after I was done, then after awhile the message popped up when I trid to render on everything in the sequence. I was able after a few tries to get my audio to render after importing an audio clip, but that's it.

When I open other sequences (I have one for each scene) I could work on and render the footage fine. I'd rather not have to re-input all the footage in that sequence, as I was almost finished with that scene.

I have my footage in ProRess 422 in Final Cut Pro. I'm working on a MacBook Pro with 4gb of RAM and the footage is on an external device with 1TB of space, connected to my computer with a FireWire 800 cable. I have had the external drive for a few weeks and have edited most of my footage with it, so I don't think that's the problem.

Thanks in advance for your help!